Element with an electrically adjustable surface emissivity for infrared radiation

ABSTRACT

An element having an electrically adjustable thermal emissivity for infrared radiation has the following layer structure: forward IR-transparent substrate; function layer whose reflectivity for IR radiation can be changed by the embedding of hydrogen; anhydrous IR-absorptive proton conductor layer;hydrogen storage layer; and electrode layer. The surface whose emissivity is to be controlled, is covered with one or more elements according to the invention, which are electrically connected with one another and can be wired in the form of strings or arrays.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] This application is a continuation of PCT International Application No. PCT/DE99/02257, filed Jul. 2, 1999 and claims the priority of German patent document 198 40 183.3, filed Sep. 3, 1998, the disclosures of which are expressly incorporated by reference herein.

[0002] The invention relates to an element with an electrically adjustable thermal emissivity for radiation in the infrared (IR) wavelength range. Such elements are used particularly for stabilizing the thermal economy of satellites and spacecraft or in the air-conditioning control in vehicles and buildings.

[0003] German Patent Document DE 36 43 692 C2 discloses a system used for IR camouflage, in which a change of the thermal emissivity is caused by electric control of a function layer. A reversible electrochemical reaction stimulated by electric voltage signals changes the optical behavior of the function layer in the infrared wavelength range. As an example of an application, an electrochromic IR absorber cell is described which consists of a controllable polymer layer, a porous gold electrode, a lithium-conducting electrolyte, a storage layer and a back electrode. The charge carriers—lithium ions or protons, which cause the electro-chemical reaction, have to be added to the system during the manufacturing in the storage layer provided as a reservoir. During the switching of the system between different emissivity values, this reservoir is alternately emptied and filled.

[0004] The lifetime of up to 10⁵ switching cycles required in practice, in this system, can be achieved only by way of an extremely high electro-chemical current efficiency of almost 100%. Systems which meet this requirement can be implemented only by means of highly purified starting substances as well as an electrode geometry which is to be implemented at very high expenditures. In practice, therefore, it cannot be implemented into a technical production.

[0005] Another disadvantage of this element is its low switching rate. The complete transition of the arrangement from one emission condition to another, particularly at low temperatures, lasts several minutes. The reason may be the low ionic conductivity of the polymer electrolyte as well as the low diffusion rate of the ions in the function layer.

[0006] One object of the invention is to provide an element of the above-mentioned type in which the thermal emissivity can be changed rapidly, and with low switching energy.

[0007] This and other objects and advantages are achieved by the emissive element according to the invention, which has the following layer structure:

[0008] Forward IR-transparent substrate;

[0009] function layer whose reflectivity for IR radiation can be changed by the embedding of hydrogen;

[0010] anhydrous IR-absorptive proton conductor layer;

[0011] hydrogen storage layer;

[0012] electrode layer.

[0013] For this purpose, the surface whose emissivity is to be controlled, is covered with one or more elements according to the invention, which are electrically connected with one another and can be wired in the form of strings or arrays.

[0014] In the present application, the term “rear” or “rearward”, with respect to the position of a layer, refers to the side of the element which is situated toward the surface (whose emissivity is to be controlled). Correspondingly, the term “forward” indicates the side of the element which is situated toward the space in which the IR radiation occurs.

[0015] In an advantageous embodiment, the element according to the invention is closed off by a rearward substrate to which the electrode layer and the storage layer are applied.

[0016] In another embodiment, the forward substrate can carry an antireflection coating or an antireflection coating system consisting of several layers which forms the forward end of the element and which has an antireflection effect in the IR wavelength range. Specifically for the thermal control of the spacecraft, the antireflection coating carries out two functions simultaneously. It has an antireflection effect in the IR range (wavelength typically 10 μm) and has a reflection effect in the visible range to near-infrared (300 nm to approximately 2 μm).

[0017] In addition, a layer can be provided between the function layer and the proton conductor layer, for protecting the material of the function layer against oxidation.

[0018] The element according to the invention permits a continuous and reversible variation of the emissivity of a surface, for infrared radiation in a wavelength range of from 1 μm to 30 μm. The control takes place by applying an electric voltage between the function layer and the electrode layer.

[0019] The preferred field of application of the element according to the invention is the sensitive control of heat absorption or emission of a surface via radiation, for example, for low-power stabilization of the thermal economy of satellites and spacecraft or for a use in the air-conditioning control in vehicles and buildings.

[0020] The element according to the invention has the following advantages:

[0021] The function layer, with the oxidation protection layer applied thereto, simultaneously forms the forward electrode, permitting a homogeneous integral-surface rapid switching operation of the element;

[0022] only low switching energies of <1Wh/m² are required;

[0023] The achievable variation of the thermal emissivity of the elements is determined almost entirely by the antireflection coating of the outer surface of the forward substrate;

[0024] The predoping of the hydrogen storage layer with hydrogen can take place particularly effectively by an electro-chemical embedding.

[0025] Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIGS. 1 and 2 show representative embodiments of the element according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 illustrates a first embodiment of the element according to the invention. It comprises the following layer structure:

[0028] An electrode layer 2 consisting of an electrically conductive material, such as Au, Al, Pt, Pd, Cu, or a conductive oxide, such as ITO;

[0029] a proton storage layer 3, such as WO₃, Y, NiO, LaNi₅, FeTi, Pd etc.;

[0030] an anhydrous IR-absorptive proton conductor 4, preferably containing mobile proton carriers, such as imidazole or pyrazole. The proton conductor layer can consist, for example, of sulfonated polyetherketone (PEK) or polyetheretherketone (PEEK) or polyaryletherketone (PAEK).

[0031] a function layer 5 consisting of yttrium Y or yttrium dihydride YH₂ or the hydride of another rare-earth metal (such as scandium, yttrium, lanthanum, cerium, praseodymium, neodymium). Advantageously, a thin layer (such as Pd, Pt, NbO₃, V₂O₅, etc.) for a protection against oxidation is applied to the function layer (not shown in FIG. 1).

[0032] a substrate 6 as a carrier made of a material, such as silicon, germanium, etc. which is transparent to infrared radiation;

[0033] an antireflection layer 7 or an antireflection layer system consisting of several individual layers which have an antireflection effect in the IR wavelength range.

[0034] The embodiment of the element according to the invention illustrated in FIG. 2 differs from that of FIG. 1 by the addition of a second substrate 1 made of a material, which is arbitrary—with respect to the IR transparency,—as the carrier for the rearward part of the cell. During the manufacture of the element, the function layer 5 is applied to the forward substrate 6 and the electrode layer 2 is applied to the rearward substrate 1, and the hydrogen storage layer 3 can be applied to the electrode layer 2. The two thus coated substrates can then be connected by means of the proton conductor 4 which is simultaneously used as a gluing material. Particularly sulfonated polyetherketone (PEK) or polyetheretherketone (PEEK) or polyaryletherketone (PAEK) with mobile proton carriers, such as imidazole or pyrazole, are suitable as materials for the proton conductor.

[0035] The method of operation of the embodiments of the invention is the same in both cases. If an electric voltage of typically 2 V is applied to the two electrodes 2 and 5, with the minus pole of the voltage source contacting the function layer 5, (made, for example, of YH₂), positively charged protons move through the proton conductor 4 from the storage layer 3 into the function layer 5 and are neutralized there. When a sufficiently large number of hydrogen atoms are embedded, this function layer changes into YH₃ which has semiconducting properties and is transparent to infrared radiation. In this case, incident radiation can penetrate to the proton conductor 4 which has an absorption of virtually 100%. The surface of the element is therefore highly emitting.

[0036] If the poles of the electric voltage are reversed, so that the plus pole contacting the function layer 5, protons move through the proton conductor 4 back into the storage layer and the function layer 5 is converted back to the stable YH₂, which has a metallic character and represents a wide-band mirror for infrared radiation. In this condition, the surface of the element has a low emission.

[0037] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 

What is claimed is:
 1. An element with an electrically adjustable surface emissivity for infrared radiation, comprising: a forward IR-transparent substrate; a function layer whose reflectivity for IR radiation can be changed by embedding of hydrogen; an anhydrous IR-absorptive proton conductor layer; a hydrogen storage layer; and an electrode layer.
 2. The element according to claim 1 , wherein said infrared radiation has a wavelength within a range of approximately 1 μm to approximately 30 μm.
 3. The element according to claim 1 , wherein the electrode layer is applied to a rearward substrate.
 4. The element according to claim 1 , further comprising an oxidation protection layer disposed between the function layer and the proton conductor layer.
 5. The element according to claim 4 , wherein the oxidation protection layer is made of a material selected from the group consisting of Pd, Pt, NbO₃ or V₂O₅.
 6. The element according to claim 4 , wherein the electrode layer is made of an electrically conductive material.
 7. The element according to claim 6 , wherein said electrically conductive material is selected from the group consisting of Au, Pt, Al, Pd, Cu, ITO.
 8. The element according to claim 1 , wherein the hydrogen storage layer is made of a material selected from the group consisting of WO₃, Y, NiO, LaNi₅, FeTi and Pd.
 9. The element according to claim 1 , wherein the proton conductor layer contains mobile proton carriers.
 10. The element according to claim 9 , wherein said mobile proton carriers comprise one of imidazole and pyrazole.
 11. The element according to claim 9 , wherein the proton conductor layer is made of a material selected from the group consisting of sulfonated polyetherketone, polyetheretherketone and polyaryletherketone.
 12. The element according to claim 1 , wherein the function layer is made of a material which forms at least two different hydrides, whose IR-reflectivities are different.
 13. The element according to claim 12 , wherein the function layer is made of a hydride of a rare-earth metal.
 14. The element according to claim 1 , wherein the rare earth metal is one of scandium, yttrium, lanthanum, cerium, praseodymium and neodymium.
 15. The element according to claim 14 , wherein the function layer is made of an alloy of at least two of said metals.
 16. The element according to claim 1 , wherein the forward substrate is made of a material selected from the group consisting of silicon or germanium.
 17. The element according to claim 1 , wherein the forward substrate carries one of an antireflection layer and an antireflection layer system which has an antireflection effect in the IR wavelength range.
 18. Use of an element according to claim 1 , for regulating one of thermal economy of spacecraft by the controlled radiation of heat in space, and the air-conditioning in buildings or vehicles. 